Electrode Structure for Stacked Alkaline Fuel Cells

ABSTRACT

A flat electrode structure for use in alkaline fuel cell stacks has electrolyte and gas inlet and outlet manifolds, with the gas manifolds being at the sides of the electrode. There is at least one gas inlet manifold and one gas outlet manifold in each side frame, and electrolyte and gas flow channels formed in the top and bottom, and side frame members. Side-to-side gas flow of the fuel gas or oxidizer gas across the electrode face is effected, with bottom to top electrolyte flow. In another embodiment of electrode structure, an embedded metal frame around the electrode serves as a current collector while stiffening the frame so as to reduce thermal expansion.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a Divisional application of application Ser. No. 11/004,988filed Dec. 7, 2004, which document is incorporated herein by referencein its entirety.

FIELD OF THE INVENTION

This invention relates to alkaline fuel cells, and particularly to theflat electrode structures from which a stacked alkaline fuel cell isassembled. Specifically, the present invention provides for the designof flat electrode structures for use in stacked alkaline fuel cellswhich permit efficient and low loss gas flow across gas diffusionelectrodes, and the flow of circulating alkaline electrolyte, throughthe stacked alkaline fuel cell. Another feature of the present inventionprovides for improved electrode contact with considerably reduced riskof electrode buckling during thermal cycling of the stacked alkalinefuel cell.

BACKGROUND OF THE INVENTION

Alkaline fuel cells have been known, at least in rudimentary form sinceshortly after the turn of the 20th century. Indeed, alkaline fuel cellshave found at least limited success and acceptance because of their useby NASA, particularly since the Apollo missions. Alkaline fuel cellswere also used by NASA for the space shuttle Orbiter vehicles. However,there has been much greater commercialization of Proton ElectrodeMembrane (PEM) fuel cells for a variety of reasons that need not bediscussed in detail here. On the other hand, the market is once againturning to alkaline fuel cells because of several specific advantagesthat they have over PEM fuel cells. Those advantages include the factthat alkaline fuel cells can be manufactured without having to rely onprecious or noble metal electrodes; and that the electrolyte is alkalineand not acidic, which leads to better electrochemical performance andgenerally broader operating temperatures than those of PEM fuel cells

The general structure of alkaline fuel cells is quite simple. Typically,fluid channels are formed through the plastic electrode frames for thedistribution of gas and electrolyte. Typically, the fuel gas ishydrogen, although it may also be such as methanol vapour, the oxidizergas is oxygen or air, and the electrolyte is alkaline solution such asaqueous potassium hydroxide solution. One purpose of any design ofelectrode frames for alkaline fuel cells is to provide for evendistribution of the flow of gases across the faces of the electrodes.However, the prior art alkaline fuel cells have had problems relating tothe elimination of droplets of moisture which develop in the gas path.Prior art alkaline fuel cells also have had difficulty with respect tothermal stresses that may be caused by uneven currents, typicallybecause of uneven gas flow, among other contributing factors. Thepresent invention seeks to overcome those and other shortcomings ofprior art alkaline fuel cells by providing for even distribution of theflow of gases across the face of the electrodes, and by providing designfeatures which effectively eliminate unwanted buildup of droplets ofcondensate which may be contaminated with electrolyte running down theface of the electrodes.

The present invention also provides designs which reduce thermalstresses that may be caused by uneven currents as they flow through theelectrode structures, and which are also caused by thermal cycling. Thatfeature is particularly accomplished by the provision of a metal contactframe embedded in the plastic electrode frame so as not only to improvecurrent collection in monopolar cells, but also so as to significantlyreduce the thermal expansion of the plastic frame. This reduces stressesimposed on the electrode as well as stresses imposed on the inter-cellseal, and thereby contributes to improved tolerance of thermal cycling.This, in turn, provides for increased longevity of the stacked alkalinefuel cell structure.

It will be understood by those skilled in the art that the features ofthe present invention as they are described thereafter may be equallyapplicable to monopolar cell designs and, with appropriate amendmentsand alterations as may be required, to bipolar cell designs. Those termsare meant, in this case, particularly to describe stacked alkaline fuelcell structures where monopolar cell structures employ edge currentcollection, and bipolar cell structures where bipolar plates may beemployed for cell interconnects.

The typical material from which plastic frames for flat electrodestructures for use in alkaline fuel cells are manufactured is beyond thescope of the present invention, except as will be described hereafterwith respect to the stiffness, modulus of elasticity, and coefficient ofthermal expansion, of that material. Suffice it to say that suchmaterial may be either a thermoplastic material or a thermosettingmaterial. In general, openings are formed through the thickness of theplastic frames so as to provide for passages which permit gas flow orelectrolyte flow from one end of the stack structure to the other. Astacked alkaline electric fuel cell structure is assembled by placingflat electrode structures adjacent one to another, observing polarity ofthe electrodes being put into place, and securing them by such asadhesive, compression, welding and other well-known methods.Accordingly, such a stacked structure with openings in the plasticframes is said to have internal manifolding, as opposed to externalmanifolding, so that inlet and outlet conduits for gas and electrolytecan be connected to the entire stack structure.

In the design of alkaline fuel cells which employ a circulatingelectrolyte, the electrolyte enters each cell of the stack at the bottomthereof, and flows upwardly. Exit channels formed at the top of the cellin the frame structure therefore are typically designed so as to permiteasy exit of any entrained gas bubbles there may be in the liquidelectrolyte. Moreover, as a consequence of the electrochemical reactionwhich occurs within the fuel cell, water is created in the cell, and asa result condensation will typically form in and outside the electrolytediffusion layer of any of the electrode structures. This, in turn, maylead to partial wetting and electrode “weeping”, whereby droplets ofcondensate will contaminate the electrolyte as it runs across the gasface of the electrode. Regrettably, in some extreme cases, it ispossible that electrolyte may find a path through imperfectelectrode-to-frame seals, or cracks on the electrode surface. This, inturn, may lead to electrolyte leaks.

Any liquid which finds its way into gas spaces of the cells must bepromptly removed in order to assure good access of the gas to theworking surface of the electrode. This has typically meant in prior artalkaline fuel cells that the gas would flow from top to bottom of eachof the individual cells, so as to carry the liquid out of the cell in amanner which provides for the least hydraulic resistance to the flow offluid, namely downwardly with the assistance of gravity. A typical priorart cell structure provided for flat, thin gas spaces in the individualcells, having one or a plurality of exit slits at the bottom of thecell. However, the problem has been that such bottom slits may becomeblocked by drops of liquid which remain in place as a consequence ofcapillary forces. If there is a plurality of slits, and some of thembecome blocked, then there will be an uneven distribution of gas flowacross the face of the electrode, resulting in weakened performance ofthat cell. It the main exit slit becomes blocked, then the entire cellwill malfunction.

Moreover, typical prior art stacked alkaline fuel cell structures reliedon parallel feed of gases, where the pressure differential between theinlet and outlet across any cell could be too small to overcome thecapillary forces and to blow out the offending drop of liquid. If anyone or more individual cells became blocked, such blockage might not bewell noticed in the hydraulic behaviour of the stack, even though theelectric behaviour may be compromised. This has led designers to arriveat somewhat complicated solutions in which groups of cells are cascadedso as to achieve high flow rates and high pressure differentials. Inturn, this requires additional pumping power or, when a blocked cell canbe electrically detected, increased gas flow and higher pressure for ashort period of time so as to blow out the offending liquid by force.

DESCRIPTION OF THE PRIOR ART

Several patents and a patent application publication have been noted andare referred to simply because they provide for general teachings ofalkaline fuel cell structure and electrodes, but otherwise are notrelevant to the present invention.

Ovshinsky et al US Patent Application Publication 2004/0161652,published Aug. 19, 2004, teaches an alkaline fuel cell pack havinggravity fed electrolyte circulation and water management. Here, there isa non-forced electrolyte and air stream which circulates to the fuelcell pack as a result of thermal convection resulting from heat producedin the fuel cell at the hydrogen and air electrodes. The design isintended to eliminate the need for pumping devices.

Landsman et al U.S. Pat. No. 5,480,735, issued in Jan. 2, 1996, isconcerned with the provision of electrodes for alkaline fuel cell, wherethe electrodes include a porous substrate and a catalyst layer. Thecatalyst layer includes catalyst particles, a hydrophobic binder, andhydrophilic catalytically inactive particles, whereby a network ofliquid transport pathway is provided through the catalyst layer.

Venkatesan et al U.S. Pat. No. 6,790,551, issued Sep. 14, 2004, teachesoxygen electrodes which operate through the mechanism of redox couplesin instant startup alkaline fuel cells. The redox couples providemultiple degrees of freedom in selecting the operating voltagesavailable for the fuel cells. Thus, the oxygen electrodes provide a“buffer” or “charge” of oxidizer which is available within the oxygenelectrode at all times.

Ruth et al U.S. Pat. No. 6,797,667, issued Sep. 28, 2004, provides aprocess whereby an anode catalyst for fuel cells may be prepared. Here,a platinum-ruthenium catalyst is prepared and provided, whereby a hightolerance to carbon monoxide poisoning of the fuel cell is achieved.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, there isprovided a flat electrode structure for use in alkaline fuel cellstacks, where the fuel cell stack comprises a plurality of flatelectrode structures placed side-by-side so as to have electrolyte inletand outlet manifolds, fuel gas inlet and outlet manifolds, and oxidizergas inlet and outlet manifolds throughout the length of the stack.

Each of the stacked flat electrode structures comprises a framedelectrode having an electrode face for contact with the electrolyte anda respective one of the fuel gas or the oxidizer gas. The electrode issecured in a surround frame having top and bottom frame members andopposed side frame members. The electrolyte, fuel gas, and oxidizer gasmanifolds are each respectively in fluid communication through thethickness of the frame members for external connection at the ends ofthe fuel cell stack to respective electrolyte, fuel gas, and oxidizergas conduits.

The electrolyte inlet manifold in each flat electrode structure isformed through the thickness of the bottom frame member, and theelectrolyte outlet manifold is formed through the thickness of the topframe member.

In each flat electrode structure, there is at least one fuel gas inletmanifold and one oxidizer outlet manifold formed through the thicknessof one of the side frame members, and at least one oxidizer inletmanifold and at least one fuel gas outlet manifold formed through thethickness of the other of the frame members.

Also, in each flat electrode structure, the electrolyte inlet and outletmanifolds are in fluid communication with the electrode face throughelectrolyte flow channels formed in the surface of the top and bottomframe members at the same side of the electrode structure where theelectrode face is located.

Still further, in each flat electrode structure, two electrolyte flowchannels are formed in each of the top and bottom frame members so as tobe in fluid communication with respective top and bottom corners of therespective electrode face.

Gas flow channels are formed in the surface of each of the side framemembers of each flat electrode structure, so as to provide fluidcommunication between the respective electrode face and the respectivefuel gas inlet and outlet manifolds or oxidizer inlet and outletmanifold. Thus, side-to-side gas flow of the respective fuel gas oroxidizer gas across the electrode face is effected.

The flat electrode structure may be such that the electrolyte flowchannels are straight.

However, the electrolyte flow channels may follow a convoluted path fromthe respective corners of the electrode face to the respectiveelectrolyte inlet or outlet manifold.

Indeed, typically, the convoluted path of the electrolytic flow channelsis serpentine.

Moreover, the convoluted path of the electrolyte flow channels may beconfigured so as always to accommodate an upward flow of electrolyte andthereby so as to preclude the development of gas lock caused by trappedgas bubbles in the liquid column of electrolyte within the electrolyteflow channels.

Another feature of the flat electrode structure of the present inventionis that the gas inlet and outlet manifolds are formed in each of theside frames, and their respective gas flow channels, are arranged insuch a manner that there is fluid communication among the gas flowchannels in one of the side frame members to the gas flow channels inthe other of the side frame members.

The flat electrode structure may be such that there are at least two gasinlet and outlet manifolds formed in each of said side frame members,and they are arranged in alternative order; or they may be arranged inadjacent groups.

Typically, each of the gas flow channels has a height substantiallyequal to the height of the respective gas inlet or outlet manifold withwhich it is in direct fluid communication adjacent that respective gasinlet or outlet manifold, and has a greater height than the height ofthe respective gas inlet or outlet manifold at the end of the gas flowchannel adjacent the electrode face.

The cross-section of each of the gas inlet and outlet manifolds may beessentially rectangular, having greater height than width. Moreover, thecorners of each of said gas inlet and outlet manifolds are typicallyrounded.

It is usual that the lowest elevation of the bottommost gas outletmanifold is below the elevation of the bottom edge of the electrodeface.

In another feature of the present invention, where the electrode faceincludes a metallic electric current collector member, each of the topand bottom frame members and each of the opposed side frame members areformed of a plastic material. There is a metal conductive foil memberembedded in the plastic frame members so as to form a continuousembedded metal contact frame surrounding the electrode face and being inelectrically conductive relationship to the current collector member.

The moduli of elasticity of the plastic material of the plastic framemembers, and of the metal conductive foil member, are such that themetal material of the metal conductive foil member is typically at least10 times or more stiffer than the plastic material of the plastic frame.

The plastic material of the plastic frame member may include a fillerchosen from the group consisting of talc, alumina, silica, glass,kaolin, kaolinite, calcite, carbon, ceramic fillers, and mixturesthereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features which are believed to be characteristic of thepresent invention, as to its structure, organization, use and method ofoperation, together with further objectives and advantages thereof, willbe better understood from the following drawings in which a presentlypreferred embodiment of the invention will now be illustrated by way ofexample. It is expressly understood, however, that the drawings are forthe purpose of illustration and description only and are not intended asa definition of the limits of the invention. Embodiments of thisinvention will now be described by way of example in association withthe accompanying drawings in which:

FIG. 1 is an elevation view of a typical flat electrode structure inkeeping with the present invention, showing a simple configuration ofelectrolyte flow channels;

FIG. 2A and FIG. 2B show alternative configurations of electrolyte flowchannels, it being understood that the other end of the flat electrodestructures of those figures is identical to the end which is shown;

FIG. 3 is an elevation of another typical flat electrode structure inkeeping with the present invention, showing a typical arrangement of gasflow manifolds and their associated gas flow channels;

FIG. 4 is an elevation of a further typical flat electrode structurehaving an alternative arrangement of gas flow manifolds and theirassociated gas flow channels; and

FIG. 5 is an elevation view of a further typical flat electrodestructure in keeping with another feature of the present inventionwhereby deformation as a consequence of thermal cycling is alleviated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The novel features which are believed to be characteristic of thepresent invention, as to its structure, organization, use and method ofoperation, together with further objectives and advantages thereof, willbe better understood from the following discussion.

Turning first to FIG. 1, a first typical embodiment of a flat electrodestructure which is suitable for use in alkaline fuel cells is shown at12. Other typical embodiments of flat electrode structures which aresuitable for use in alkaline fuel cells are shown at 14 in FIG. 2A and16 in FIG. 2B. However, the same reference numerals are used throughoutall of the figures of drawings which are described hereafter to indicatethe same feature of the flat electrode structures being discussed at anytime.

It will also be understood that the embodiments shown in FIGS. 1, 2A,and 2B, are intended only to show representative electrolyte inlet andoutlet manifolds and electrolyte flow channels; and likewise, theembodiments shown in FIGS. 3 and 4 are intended only to show typicalarrangements of fuel gas and oxidizer gas inlet and outlet manifolds andtheir associated gas flow channels. In other words, each of thosefigures has been highly simplified for purposes of clarity andillustration only.

Each flat electrode structure comprises a framed electrode which isshown generally at 20. Typically, the electrode structure isrectangular. The specific features, chemistry, and structure, of theelectrodes 20 are outside the scope of this present invention. Whileelectrolyte flow from the bottom to the top of the cell is known, theinventor herein has quite unexpectedly discovered that better and moreefficient fuel cell operation is achieved when the gas flow of the fuelgas and oxidizer gases is horizontal, that is from side to side of eachrespective cell, across the electrode face of that cell. This isdescribed in greater detail hereafter.

Of course, it will be understood that each electrode will have a workingface that is designed in keeping with well known principles to interactwith the electrolyte or the respective fuel gas or oxidizer gas.

Each electrode frame which surrounds the electrode has top and bottomframe members 22 and 24, and opposed side frame members 26 and 28.Located in the top frame member 22 is the electrolyte outlet manifold32, which is formed through the thickness of the electrode frame. Theelectrolyte inlet manifold 34 is formed in the bottom frame member 24.Thus, it will be understood that electrolyte flow in the cell across theelectrode face will be from bottom to top of the cell. Two inletchannels 36 are formed in each of the top and bottom frame members 22,24, and are referred to herein as electrolyte flow channels. It will beseen that the electrolyte flow channels are in fluid communicationacross the electrode face through flow channel faces 40. It will also beseen that the electrolyte flow channels communicate to the respectiveinlet and outlet manifolds 34 and 32 from the corners of the electrodeface. Because an electrolyte flow within the cell is achieved as aconsequence of both pumping and convection flow, wetting of the entireelectrode face is assured. It will also be understood that the cornerexits for the electrolyte into the electrolyte flow channels will helpin the easy removal of entrained gas bubbles within the liquidelectrolyte, even if the cell is leaned out of its vertical position.

It is known that electrolyte flow channels which are connected to commonmanifolds may present paths for parasitic currents. In order to keepthose parasitic currents to a minimum, the resistance of the electrolytechannels should be reasonably high. The electrical resistance of theliquid column of electrolyte within an electrolyte flow channel isdirectly proportional to the length of the flow channel and inverselyproportional to its cross-section. However, there may also be hydraulicconsiderations which limit the design choice as to how small theelectrolyte flow channels may be, so it may be considered to bedesirable to increase the length of the electrolyte flow channels byarranging them in a convoluted path. Typically, that path may beserpentine, as shown at flow channels 36A in FIG. 2A, and flow channels38B in FIG. 2B. Of course, it has been noted that the opposite ends ofthe electrodes 14 and 16 shown in FIGS. 2A and 2B, respectively, will bethe same as the end which is shown. The specific difference between theelectrolyte flow channels 36A and 38B is that channels 38B are nearlytwice as long as channels 36A. In any event, all of the electrolyte flowchannels are in fluid communication with the respective electrolyteinlet and outlet manifolds 34 and 32.

It will also be understood that the design of any of the electrolyteflow channels is such that there is always an upward flow of electrolytethrough the respective electrolyte flow channel so as to therebypreclude the development of any gas lock which might occur as aconsequence of trapped gas bubbles in the liquid column of electrolytewithin the electrolyte flow channels.

Turning now to FIG. 3, another typical configuration for a flatelectrode structure in keeping with the present invention is shown.Here, for purposes of simplicity and clarity, consideration has not beengiven to the electrolyte flow channels and their respective inlet andoutlet manifolds. Thus, the purpose of the following discussion is toexplain the layout of gas manifolds in a fuel cell stack, and to showgas flow across the electrode face from side to side of the electrode.

The configuration of the embodiment of FIG. 3 comprises the same top andbottom frame members 22, 24 and side members 26, 28, which surround theelectrode 20. What is shown in this figure particularly is gas flow ofthe fuel gas, which is the consumable fuel for the stacked alkaline fuelcell. In the embodiment shown, there are a plurality of fuel gas inletmanifolds 46 formed through the thickness of the right side frame member28, and a plurality of fuel gas outlet manifolds 48 formed through thethickness of the left side frame member 26. Gas flow across the face ofthe electrode 20 is seen to be from right to left as shown by arrows 50in this illustrative embodiment.

Moreover, it will be understood that there is fluid communication amongthe inlet gas manifolds 46 and the outlet gas manifolds 48, although thegas flow across the face of the electrode 20 tends to be linear andlaminar. It will also be understood that, in some circumstance, theremay be only a single gas flow manifold for each of the fuel gas andoxidizer gas formed through the thickness of the side frame members ateach side of the electrode frame structure.

It is also seen in FIG. 3 that there are a plurality of oxidizer gasinlet manifolds 56 formed through the thickness of the left side framemember 26, and a plurality of oxidizer gas outlet manifolds 58 formedthrough the thickness of the right side frame member 26. Those skilledin the art will understand, of course, that the oxidizer gas side of theelectrode, or more particularly an oxidizer gas electrode frame havingthe appropriate electrode therein, will be on the back side of theelectrode frame 44. It will also be understood, of course, that theoxidizer gas flow will be similar to that which is shown in FIG. 3 butin the opposite direction, that is from left to right as seen in thefigure.

FIG. 3 shows the fuel gas inlet and outlet manifolds and the oxidizergas inlet and outlet manifolds being arranged in alternative order. Thatis, between a pair of fuel gas inlet manifolds 46 there is formedthrough the thickness of the right side frame 28 an oxidizer gas outletmanifold 58. Inspection shows the same arrangement on the left sideframe 26, but in the reverse order so that the topmost manifold formedthrough the thickness of the left side frame member 26 is an oxidizergas inlet manifold, and the topmost manifold formed through thethickness of the right side frame member 28 is a fuel gas inletmanifold, with the bottommost manifolds formed through the thicknessesof the left and right side frame members 26 and 28 being a fuel gasoutlet manifold and an oxidizer gas outlet manifold, respectively.

Referring briefly to FIG. 4, an alternative arrangement for the inletand outlet gas manifolds for the fuel gas and for the oxidizer gas isshown. Here, the two fuel gas inlet manifolds 46 are shown as beingadjacent to one another in the right side frame member 28, and the twooxidizer gas outlet manifolds 58 are also shown as being adjacent one toanother in the right side frame member 28. A similar arrangement is madein the left side frame member 26 for the oxidizer gas inlet manifolds 56and the fuel gas outlet manifolds 48. Otherwise, the same principlesapply as to the functionality of the structure as it relates to both theelectrolyte flow manifolds and the gas flow manifolds.

However, a further feature is also shown in FIGS. 3 and 4. What is shownin those figures are gas flow channels 60 and 62, which, in this case,are the gas flow channels which provide for fuel gas flow from the fuelgas inlet manifolds 46 to the fuel gas outlet manifolds 48. It will beunderstood, once again, that there is fluid communication among theinlet fuel gas flow channels 60 and the outlet fuel gas flow channels62, and that the fuel gas flow is essentially linear and laminar acrossthe electrode face.

It will also be understood that the horizontal flow of gas in the cellwill not significantly affect the flow of liquid effluent. Droplets ofcondensate which may be contaminated with electrolyte will eventuallyfind their way to the bottom of the cell through the diffuser mat 66,which is shown for purposes of this discussion in FIG. 3. In the case ofa bipolar plate, the liquid will find its way to the bottom of the cellthrough gas spaces found in a bipolar plate.

What is important to note is that the design and placement of the gasmanifolds, particularly the bottommost gas manifolds, and of the gasflow channels formed in the respective left and right side framemembers, are arranged so as to assure that what liquid collects at thebottom of the cell will eventually find its way out of the stack. Thisis accomplished by the fact that the lowest elevation of the bottommostgas outlet manifolds 48, 58 are below the elevation of the bottom edgeof the electrode face 20.

Other design features are also provided. They include the fact that eachof the respective gas inlet and outlet manifolds 46, 48, 56, 58 isconfigured so as to have a greater height than width. Moreover,typically the corners of each of the gas inlet and outlet manifolds arerounded; and this assures liquid flow particularly from the bottommostgas outlet manifolds 48, 58.

The design of each of the gas flow channels provides diffuser effect.This is accomplished by having the height of the gas flow channels to beessentially the same as the height of the respective gas flow manifoldwith which they are in direct fluid communication. However, the otherend of each of the gas flow channels which is adjacent the electrodeface has a greater height than the manifold end of the gas flowchannels. This has the salutary effect of providing for a more evenlydistributed gas flow across the entire height of the electrode face,while reducing the exit pressure and speed of the fuel gas or oxidizergas as they flow from the respective gas inlet manifolds 46 or 56.

The gas flow channels 60A and 62A, as they are shown in FIG. 4, are seento have splitters 70, by which more linear flow of the gas is assured.Moreover, the arrangement of the gas flow channels as shown in FIG. 4accommodates the arrangement where the inlet and outlet gas manifoldsare arranged in adjacent pairs, which simplifies the design of the endplates for the stack of flat electrode structures in keeping with thepresent invention.

Typically, the depth of the gas flow channels as they are formed in thefaces of the respective side frame members of the electrode structuresin keeping with the present invention may be in the range of from 0.5 to1.0 mm. Thus, the increased height of the gas flow channels in the areain adjacent the electrode face will be understood to effect gas flow ina favorable manner.

Moreover, any liquid which may collect and be retained in the gas flowchannels as a consequence of capillary action will, in any event, sit atthe bottom of the gas flow channel. Because the gas flow is directedhorizontally, what liquid may be collected and retained will be at thebottom of the channels; and it will be understood that the height of thechannels will be significantly greater than the “capillary elevation”which is a function of the wetting properties of the liquid on thecapillary wall and the dimensions of the capillary. In practical terms,this means that all of the manifolds with the exception of the twolowest outlet manifolds will remain dry and unobscured most of the time.Indeed, while most previous designs of stacked alkaline fuel cells willreluctantly accommodate less then 50% blockage of gas manifolds, andcells fail when blockage exceeds that percentage, it has been observedthat there is typically considerably less than 20% blockage of only thebottommost gas outlet manifolds and no blockage of higher manifolds instacked alkaline fuel cells in keeping with the present invention.

It has also been observed that operation of a stacked alkaline fuel cellhaving electrode structures more or less in keeping with theconfiguration of FIG. 4 has shown remarkably good gas flow distribution,with a 60% performance improvement over earlier top-to-bottom designs.

Referring now to FIG. 5, an improved electrode structure is shown whichwill significantly reduce thermal expansion of the electrode cellstructure during thermal cycling of the fuel cell, and which therebyreduces the stresses that are imposed on the electrode and its seal.

Typically, a monopolar flat electrode structure has a current collectorwhich is usually a metal screen or mesh, and in prior art designs, thatcurrent collector would extend through the plastic frame to the outside.Another arrangement has been to provide a single metal contact embeddedin the frame and attached to one side of the electrode. No matter whatarrangement was made, there was a compromise between resistive losses inthe current collector, the weight of the current collector, and itscost.

Still further, the plastic material of the frame which surrounds theelectrode may exhibit several times higher coefficient of thermalexpansion than the electrode material itself. It will be seen,therefore, that changes of temperature during thermal cycling could leadto stressing the bond and seal between the electrode and the frame,which in turn would eventually lead to cracks in the electrode orbuckling of the electrode, and in any event to premature failure. Itwill be kept in mind that the relative strength of the frame is muchgreater than that of the rather delicate mesh in the electrode, so thatstretching of the electrode at increased temperature beyond its limit ofelasticity would lead to buckling when the electrode cools.

It will also be kept in mind that the plastic material of the framestructures may have a filler, such as talc, alumina, silica, glass,kaolin, kaolinite, calcite, carbon, ceramic fillers, and mixturesthereof.

FIG. 5 shows an electrode structure 74 having a configuration which is,in general, similar to that of FIG. 4. However, this electrode structurefurther includes a conductive metal foil member 76, which is typicallycopper but may be other electrically conductive metals, and which isembedded within the plastic frame member 22, 24, 26, 28. It will be seenthat the conductive foil metal member 76 is a continuous embedded metalcontact frame which surrounds the electrode face. It will be understoodthat the embedded metal contact frame is in electrically conductiverelationship to the current collector member of the electrode 20, and isattached thereto by such means as spot welding, soldering, swaging, andso on as is well known to those skilled in the art. In any event, thepresence of the continuous embedded metal contact frame 76 provides formuch improved current collection and reduced resistive losses. Also, inkeeping with a feature of the present invention whereby thermalstability of the electrode structure is achieved, the presence of thecontinuous embedded metal contact frame 76 provides for improvedmechanical stability during thermal cycling, thereby resulting inreduced wear and longer life.

Indeed, a relatively small metal element will achieve the desiredeffect. For example:

The modulus of elasticity of the metal contact material is 1.15×10⁶kg/cm², and the modulus of elasticity of the plastic material of theframe is 0.018×10⁶ kg/cm². The ratio of the two is 1.15/0.018=63.9. Thatleads to the conclusion that the metal material is approximately 64times stiffer than the plastic material.

Moreover, the coefficient of thermal expansion of the plastic materialis 70 ppm/degree C.; and the coefficient of thermal expansion of themetal material is approximately 16 ppm/degree C. Even if the metal andplastic materials were to be warmed up by 1° C., then the metal willincrease its length by approximately 16 ppm, while the plastic willincrease its length by approximately 70 ppm. Since the continuous metalcontact frame 76 is embedded in the plastic frame member 22, 24, 26, 28,they are bonded one to the other. Thus, the metal will restrict theelongation of the plastic, and the plastic will pull or to try tostretch the metal, until such time as they reach a compromise orequilibrium.

It can be shown that if the metal part stretches by Δe₁ and the plasticcompresses by Δe₂, then the following ratio is relevant:$\frac{\Delta\quad e_{1}}{\Delta\quad e_{2}} = \frac{A_{2}M_{2}}{A_{1}M_{1}}$where A₁, A₂ are the cross-sectional areas of the two components, andM₁, M₂ are their respective moduli of elasticity. At the same time, thedifference of unrestricted thermal elongation of the two componentsequals the sum of the elastic elongation of the first part plus elasticcompression of the second part, giving rise to the followingrelationships:Δe=e ₂ −e ₁ =Δe ₁ +Δe ₂Δe=70−16=54 ppm

In an exemplary electrode structure in keeping with the presentinvention, the cross-section of the plastic top and bottom framemembers, and the side frame members, is 20×2=40 mm^(2;) and thecross-section the conductive foil metal member is 8×0.5=4 mm². Then theratio is:$\frac{\Delta\quad e_{1}}{\Delta\quad e_{2}} = {\frac{40 \times 0.018 \times 10^{6}}{4 \times 1.15 \times 10^{6}} = 0.156}$

In other words, the material of the plastic top and bottom framemembers, and the side frame members, will yield six times more than therelatively thin conductive foil metal member. Further calculationyields:Δe₁=7.3 ppm and Δe₂=46.7 ppm

The net result is that the composite part will elongate only by16+7.3=70−46.7=23.3 ppm,which is compatible with the material of the electrode over a wide rangeof temperatures.

Other modifications and alterations may be used in the design andmanufacture of the fuel cell electrode structures of the presentinvention without departing from the spirit and scope of theaccompanying claims.

Throughout this specification and the claims which follow, unless thecontext requires otherwise, the word “comprise”, and variations such as“comprises” or “comprising”, will be understood to imply the inclusionof a stated integer or step or group of integers or steps but not to theexclusion of any other integer or step or group of integers or steps.

1. A flat electrode structure for use in alkaline fuel cell stacks,where the fuel cell stack comprises a plurality of flat electrodestructures placed side-by-side so as to have electrolyte inlet andoutlet manifolds, fuel gas inlet and outlet manifolds, and oxidizer gasinlet and outlet manifolds throughout the length of the stack; each ofsaid stacked flat electrode structures comprising a framed electrodehaving an electrode face for contact with said electrolyte and arespective one of said fuel gas or said oxidizer gas, and being securedin a surround frame having top and bottom frame members and opposed sideframe members, and wherein the electrolyte, fuel gas, and oxidizer gasmanifolds are each respectively in fluid communication through thethickness of the frame members for external connection at the ends ofthe fuel cell stack to respective electrolyte, fuel gas, and oxidizergas conduits; wherein the electrolyte inlet manifold in each fiatelectrode structure is formed through the thickness of the bottom framemember, and the electrolyte outlet manifold is formed through thethickness of the top frame member; wherein, in each flat electrodestructure, there is at least one fuel gas inlet manifold and at leastone oxidizer gas outlet manifold formed through the thickness of one ofsaid side frame members, and at least one oxidizer gas inlet manifoldand at least one fuel gas outlet manifold formed through the thicknessof the other of said frame members; wherein said electrode face includesa metallic electric current collector member; and wherein each of saidtop and bottom frame members and said opposed side frame members areformed of a plastic material, and wherein there is a metal conductivefoil member embedded therein so as to form a continuous embedded metalcontact frame surrounding said electrode face and being in electricallyconductive relationship to said current collector member.
 2. The flatelectrode structure of claim 1, wherein said framed electrode isrectangular.
 3. The flat electrode structure of claim 1, wherein themoduli of elasticity of the plastic material of said plastic framemembers and of said metal conductive foil member are such that the metalmaterial of said metal conductive foil member is at least 10 timesstiffer than the plastic material of said plastic frame.
 4. The flatelectrode structure of claim 1, wherein the plastic material of saidplastic frame member includes a filler chosen from the group consistingof talc, alumina, silica, glass, kaolin, kaolinite, calcite, carbon,ceramic fillers, and mixtures thereof.